A liquid crystal display has become widely available as an information display because of its advantages, such as light weight, thinness, and low power consumption. On the other hand, full gray-scale display is demanded on the display device side as a volume of information transmission media increases or processing ability of computer hardware improves. Thus, the full gray-scale display is essential to the liquid crystal display as well to achieve further widespread use.
A TFT (Thin Film Transistor) type liquid crystal display, known as one type of the liquid crystal displays, is provided with thin film transistors for individual pixels which form a display, and the gray-scale state of the liquid crystal is controlled by these transistors. Generally, line electrodes are scanned per line to open the gate of each transistor provided for individual pixels belonging to the line being scanned, and the half-tone is controlled with a peak value of a voltage applied to the source (drain) at the scanning.
On the other hand, a ferroelectric liquid crystal display has been receiving attention because the ferroelectric liquid crystal has a memory property (bistability), and therefore, it can attain high-quality display without adding active elements such as transistors, but by adopting a so-called passive matrix arrangement.
However, since the ferroelectric liquid crystal can switch between only two states in effect, it has been said that it is difficult to realize the half-tone display on the liquid crystal display using the ferroelectric liquid crystal. To eliminate this problem, the use of the dither method (spacial dividing method, temporal dividing method), and a method (analog method) of letting two switching states coexist and the like have been under active study.
However, unlike the other types of displays, the temperature dependency of the liquid crystal material characteristics is large in the liquid crystal display, and therefore, there rises a problem that its gray-scale display ability is affected considerably by the circumstances in which the display is used, especially temperature. This problem becomes particularly noticeable in a ferroelectric liquid crystal display having an unstable half-tone display state (coexistence of two stable states), and whose liquid crystal material characteristics has very large temperature dependency.
Also, the ferroelectric liquid crystal display often causes uneven display due to the characteristic distribution in the panel or the like. In other words, the uneven display occurs due to the variation in temperature and variation of characteristics in the panel. Especially in analog method which uses coexisting two switching states, thickness variation of liquid crystal layer (or cell spacing) gives large variation of characteristics in the panel. The coexisting states are quite sensitive to thickness variation of cell spacing.
A method of solving the above problem in the display (ferroelectric liquid crystal display and the like) having a bistable state is disclosed in Japanese Laid-open Patent Application Nos. 27719/1993 (Tokukaihei No. 5-27719) and 27720/1993 (Tokukaihei No. 5-27720).
In a liquid crystal display disclosed in the above publications, as shown in FIG. 8, one pixel P is divided into two sub-pixels P.sub.A and P.sub.B. Of these two pixels P.sub.A and P.sub.B, the sub-pixel P.sub.A is fully written into a first stable state with a first writing pulse, after which it is written into a second stable state corresponding to a display scale with a second writing pulse, while the sub-pixel P.sub.B is fully written into the second stable state with the first writing pulse, after which it is written into the first stable state corresponding to a display scale with the second writing pulse. In other words, the sub-pixels P.sub.A and P.sub.B respond optically in the opposite manners to the identical writing pulses.
FIG. 9 illustrates the optical response characteristics (transmittance) of the sub-pixels P.sub.A and P.sub.B forming one pixel in response to the writing pulse. In the drawing, Graph a shows the characteristics of the sub-pixel P.sub.A and Graph b shows the transmittance of the sub-pixel P.sub.B. Also, in the drawing, Graphs a' and b' indicated as a broken line respectively show the transmittance of the sub-pixels P.sub.A and P.sub.B when the ambient temperature has changed.
As it is understood from the drawing, the optical response characteristics, that is, transmittance in response to a voltage, of the sub-pixels P.sub.A and P.sub.B shift in the directions opposite to each other as the temperature changes. To be more specific, the comparison between Graphs a and a' reveals that the transmittance of the sub-pixel P.sub.A shifts in an increasing direction as the temperature changes. On the other hand, the comparison between Graphs b and b' reveals that the transmittance of the sub-pixel P.sub.B shifts in a decreasing direction as the temperature changes.
According to the conventional method, for example, to achieve half-tone transmittance I.sub.4 at the pixel P, a voltage V.sub.A and a voltage V.sub.B are applied to the sub-pixels P.sub.A and P.sub.B, respectively. Then, the sub-pixel P.sub.A shows the transmittance I.sub.4 while the other sub-pixel P.sub.B also shows the transmittance I.sub.4, thereby making it possible to attain the desired transmission I.sub.4 at the pixel P as a whole.
When the optical response characteristics have shifted with a change in temperature or the like, as is understood from FIG. 9, the sub-pixel P.sub.A attains transmittance I.sub.4 +.DELTA.I while the sub-pixel P.sub.B attains transmittance I.sub.4 -.DELTA.I upon application of the identical voltages V.sub.A and V.sub.B, respectively. Thus, the pixel P composed of the sub-pixels P.sub.A and P.sub.B attains the transmittance I.sub.4 as a whole as it does in the above case. In other words, according to the conventional method, the variance in the optical response characteristics caused by the variance in temperature or the variance of characteristics can be compensated.
However, according to the method disclosed in aforementioned Japanese Laid-open Patent Application Nos. 27719/1993 (Tokukaihei No. 5-27719) and 27720/1993 (Tokukaihei No. 5-27720), one pixel must be divided to, for example, two sub-pixels. Thus, if the resolution of the conventional display is to be secured, for example, the sub-pixels, half in size and double in number compared with the pixels in the conventional display, are necessary.
Thus, finer electrode work is demanded compared with the conventional display, which causes the cost to increase. Also, since the number of the electrode outputs of the scanning electrodes is increased twice, the two-fold scanning drivers are necessary, which also causes the cost to increase.
If the second writing pulse is applied to the two sub-pixels simultaneously to shorten the selection period when the stable state corresponding to a certain half-tone is written with the second writing pulse, the two sub-pixels demand not only their own line electrodes, but also their own column electrodes. Thus, the electrodes demand very fine work; moreover, the number of the information signal drivers must be increased twice.